The present invention relates to an organic electroluminescent (EL) device, and more precisely, to a blue-emitting, organic EL device having a long life and having high luminous efficiency and good thermal stability.
As being self-luminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light emitters is being widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as light emitters in the coming generations, since they require a greatly reduced level of voltage, they can be easily small-sized, they consume small electric power, they can emit light in a mode of plane emission, and they can easily emit three primary colors.
Known are various structures of organic EL devices having a basic constitution of positive electrode/organic light-emitting layer/negative electrode and optionally provided with a hole injection and transportation layer and an electron injection layer, such as positive electrode/hole transportation layer/organic light-emitting layer/negative electrode, and positive electrode/hole transportation layer/organic light-emitting layer/electron injection layer/negative electrode, etc.
For such organic EL devices, for example, metal complexes of phenolato-substituted 8-hydroxyquinolines have been disclosed as blue-emitting materials (see Japanese Patent Application Laid-Open No. 5-198378). However, these are problematic in that their luminous efficiency is low to be at most 0.2 lumen(lm)/W or so. The reason is because the fluorescence quantum efficiency of the host compounds is low to be at most 0.07 or so. Doping of fluorescent substances into the host compounds is effective in prolonging the life of the doped host compounds, but is not in improving their luminous efficiency.
As luminous materials (host materials) for organic EL devices capable of emitting blue at high efficiency, distyrylarylene compounds have been disclosed (see Japanese Patent Application Laid-Open No. 2-247278). In addition, it has been disclosed that the doping of organic host compounds with fluorescent substances produces the improvement in the luminous efficiency of the doped organic host compounds while prolonging their life (see International Patent Application Laid-Open No. 94/06157).
Many of those blue-emitting compounds generally have a low glass transition temperature (Tg), since xcfx80-electrons are distributed in narrow regions therein and since they have a low molecular weight. Therefore, organic EL devices comprising those compounds are problematic in that their thermal stability is poor. Organic EL devices for outdoor applications or applications in vehicles require high-temperature storage stability generally at 75xc2x0 C. or so. However, conventional organic EL devices are problematic in those applications in that, when they are kept at high temperatures of 75xc2x0 C. or so, the color to be emitted by them varies and their luminous efficiency is lowered. For these reasons, the applications of organic EL devices are inevitably limited.
Given that situation, various studies have been made in order to improve the thermal stability of organic EL devices. One example is to modify luminous materials to have dimer or oligomer structures, thereby making the materials have an elevated glass transition temperature. For this, referred to is Japanese Patent Application Laid-Open No. 8-12600, which discloses compounds (phenylanthracene derivatives) having a glass transition temperature of 181xc2x0 C. In this publication, they tried to improve the efficiency of their devices and to prolong the life thereof by mixing the hole transportation layer and the light-emitting layer. However, the devices disclosed have a luminous efficiency of at most 0.6 lm/W, which is lower than 1 lm/W, and the capacity of the devices is not satisfactory.
As has been mentioned hereinabove, no blue-emitting, organic EL devices having a long life, high efficiency and good thermal stability, which are all indispensable in their practical use, is unknown.
Given this situation, the object of the invention is to provide a practical, blue-emitting, organic EL device having a long life and having high luminous efficiency and good thermal stability.
We, the present inventors have assiduously studied in order to obtain an organic EL device having those favorable properties, and, as a result, have found that an organic EL device, in which the organic blue-emitting layer comprises an organic host compound having a specific fluorescence quantum efficiency and a fluorescent substance, and the organic host compound and the fluorescent substance are selected such that the device retains a monomeric blue-emitting ability, and in which the organic compound layers constituting the device have a specific glass transition temperature, meets the requirements. The invention has been attained on the basis of these findings.
Specifically, the invention provides an organic EL device comprising organic compound layers, of which at least one is an organic blue-emitting layer, as sandwiched between a pair of electrodes, which is characterized in that (1) the organic blue-emitting layer comprises an organic host compound having a fluorescence quantum efficiency of not smaller than 0.3 in a solid state, and a fluorescent substance, and the organic host compound and the fluorescent substance are selected such that the device retains a monomeric blue-emitting ability, and (2) all the organic compound layers have a glass transition temperature of not lower than 75xc2x0 C., while the organic compound layers adjacent to the organic blue-emitting layer have a glass transition temperature of not lower than 105xc2x0 C.
The organic EL device of the invention has an organic blue-emitting layer comprising an organic host compound and a fluorescent substance.
The organic host compound which is one component constituting the organic blue-emitting layer is not specifically defined, provided that its function is such that holes and electrons are injected thereinto and are transported therethrough to be recombined together to give off fluorescence, that its fluorescence quantum efficiency is not smaller than 0.3, and that it forms, along with the fluorescent substance, the organic blue-emitting layer having a glass transition temperature of not lower than 75xc2x0 C. Therefore, various compounds are employable herein.
For example, the organic host compound may be selected from distyrylarylene derivatives of a general formula (I): 
In formula (I), k, m and n each are 0 or 1, and (k+m+n)xe2x89xa71. In the formula, R1 to R12 each independently represent a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 6 carbon atoms, an aryloxy group having from 6 to 18 carbon atoms, an aryl group having from 6 to 20 carbon atoms, an amino group, an alkylamino group, an arylamino group, a cyano group, a nitro group, a hydroxyl group, a halogen atom, or a group of: 
The alkyl group having from 1 to 6 carbon atoms includes, for example, a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, a sec-butyl group, a t-butyl group, an i-pentyl group, a t-pentyl group, a neopentyl group, an n-hexyl group, and an i-hexyl group. The alkoxy group having from 1 to 6 carbon atoms includes, for example, a methoxy group, an ethoxy group, an n-propoxy group, an i-propoxy group, an n-butyloxy group, an i-butyloxy group, a sec-butyloxy group, an i-pentyloxy group, a t-pentyloxy group, and an n-hexyloxy group. The aryloxy group having from 6 to 18 carbon atoms includes, for example, a phenoxy group, and a naphthyloxy group. The aryl group having from 6 to 20 carbon atoms includes, for example, a phenyl group, and a naphthyl group. The amino group is represented by xe2x80x94NH2; the alkylamino group is by xe2x80x94NHR or xe2x80x94NR2 (where R indicates an alkyl group having from 1 to 6 carbon atoms); and the arylamino group is by xe2x80x94NHAr or xe2x80x94NAr2 (where Ar indicates an aryl group having from 6 to 20 carbon atoms).
The halogen atom includes, for example, a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom.
Where k=1 and m=n=0 in formula (I), the substituents in at least one combination of R1 and R2, R3 and R4, R5 and R6, R7 and R8, R9 and R10, and R11 and R12 are bonded to each other to form a saturated or unsaturated, 5-membered or 6-membered ring. In that case, the substituents maybe bonded via a hetero atom (N, O, S) to form the ring. As specific examples where R1 and R2, R9 and R10, and R5 and R6 each are bonded to each other to form an unsaturated 6-membered ring, mentioned are compounds of the following formula: 
As specific examples where R7 and R8 are bonded to each other via a hetero atom O to form a saturated 5-membered ring, R11 and R12 are bonded to each other via a hetero atom N to form a saturated 5-membered ring, and R3 and R4, and R9 and R10 each are bonded to each other to form a saturated 6-membered ring, mentioned are compounds of the following formula: 
Where k=m=1 and n=0 in formula (I), the compounds are represented by the following formula: 
wherein R3xe2x80x2, R4xe2x80x2, R9xe2x80x2 and R10xe2x80x2 each independently represent a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 6 carbon atoms, an aryloxy group having from 6 to 18 carbon atoms, an aryl group having from 6 to 20 carbon atoms, an amino group, an alkylamino group, an arylamino group, a cyano group, a nitro group, a hydroxyl group, a halogen atom, or a group of: 
and R1 to R12 have the same meanings as above.
In those, R1 and R2, R3 and R4, R3xe2x80x2 and R4xe2x80x2, R5 and R6, R7 and R8, R9 and R10, R9xe2x80x2 and R10xe2x80x2, and R11 and R12 each may be or may not be bonded to each other to form a saturated or unsaturated, 5-membered or 6-membered ring. Optionally, they may be bonded to each other via a hetero atom (N, O, S) to form the ring.
R2 and R3, R4 and R3xe2x80x2, R4xe2x80x2 and R5, R8 and R9, R10 and R9xe2x80x2, and R10xe2x80x2 and R11 each may be or may not be bonded to each other to form a saturated or unsaturated, 5-membered or 6-membered ring. Optionally, they may be bonded to each other via a hetero atom (N, O, S) to form the ring.
As specific examples where R2 and R3, R10 and R9xe2x80x2, and R4xe2x80x2 and R5 each are bonded to each other to form a saturated 5-membered ring, mentioned are compounds of the following formula: 
As those where R4 and R3xe2x80x2, and R10 and R9xe2x80x2 each are bonded to each other to form a saturated 6-membered ring, mentioned are compounds of the following formula: 
As those where R10 is a group of: 
and R9xe2x80x2 is a hydrogen atom to give a 5-membered ring, mentioned are compounds of the following formula: 
Where k=m=n=1 in formula (I), the compounds are represented by the following formula: 
wherein R3xe2x80x3, R4xe2x80x3, R9xe2x80x3 and R10xe2x80x3 each independently represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms, an alkoxy group having from 1 to 6 carbon atoms, an aryloxy group having f rom 6 to 18 carbon atoms, an aryl group having f rom 6 to 20 carbon atoms, an amino group, an alkylamino group, an arylamino group, a cyano group, a nitro group, a hydroxyl group, a halogen atom, or a group of: 
and R1 to R12, R3xe2x80x2, R4xe2x80x2, R9xe2x80x2 and R10xe2x80x2 have the same meanings as above.
In those, R1 and R2, R3 and R4, R3xe2x80x2 and R4xe2x80x2, R3xe2x80x3 and R4xe2x80x3, R5 and R6, R7 and R8, R9 and R10, R9xe2x80x2 and R10xe2x80x2, R9xe2x80x3 and R10xe2x80x3, and R11 and R12 each may be or may not be bonded to each other to form a saturated or unsaturated, 5-membered or 6-membered ring. Optionally, they may be bonded to each other via a hetero atom (N, O, S) to form the ring.
R2 and R3, R4 and R3xe2x80x2, R4xe2x80x2 and R3xe2x80x3, R4xe2x80x3 and R5, R8 and R9, R10 and R9xe2x80x2, R10xe2x80x2 and R9xe2x80x3, and R10xe2x80x3 and R11 each may be or may not be bonded to each other to form a saturated or unsaturated, 5-membered or 6-membered ring. Optionally, they may be bonded to each other via a hetero atom (N, O, S) to form the ring.
As specific examples where R8, R9, R10xe2x80x3 and R11 each are a group of: 
to form an unsaturated 5-membered ring, and R3xe2x80x2 and R4xe2x80x2 together form a saturated 5-membered ring via a hetero atom N, mentioned are compounds of the following formula: 
X and Y each independently represent an aryl group having from 6 to 20 carbon atoms, such as a substituted or unsubstituted phenyl, naphthyl, biphenyl, terphenyl, anthraryl, phenathryl, pyrenyl or perylenyl group.
The substituent includes, for example, an alkyl group having from 1 to 6 carbon atoms, such as a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, a sec-butyl group, a t-butyl group, an i-pentyl group, a t-pentyl group, a neopentyl group, an n-hexyl group, an i-hexyl group; an alkoxy group having from 1 to 6 carbon atoms, such as a methoxy group, an ethoxy group, an n-propoxy group, an i-propoxy group, an n-butyloxy group, an i-butyloxy group, a sec-butyloxy group, an i-pentyloxy group, a t-pentyloxy group, an n-hexyloxy group; an aryloxy group having from 6 to 18 carbon atoms, such as a phenoxy group, a naphthyloxy group; a phenyl group, an amino group, an alkylamino group, an arylamino group, a cyano group, a nitro group, a hydroxyl group; and a halogen atom, such as a fluorine atom, a chlorine atom, a bromine atom, an iodine atom. The group of X and Y may be substituted with one or more of those substituents.
X and Y and optionally their substituents may be bonded together to form a substituted or unsubstituted, saturated 5-membered, or saturated 6-membered ring. As specific examples of the styryl compounds where X and Y form a saturated, 5-membered or 6-membered ring, mentioned are the following compounds where X and Y form a saturated 5-membered ring, k=m=1 and n=0: 
As those where X and Y form a saturated 6-membered ring, mentioned are the following compounds: 
In the invention, the organic blue-emitting layer must indispensably have a glass transition temperature of not lower than 75xc2x0 C. Therefore, the organic host compounds to be in the layer are preferably selected from those of the following general formula (II) in which the central polyphenylene skeletons are all bonded to the adjacent ones at their para-positions: 
wherein R1 to R12, R3xe2x80x2, R4xe2x80x2, R9xe2x80x2, R10xe2x80x2, R3xe2x80x3, R4xe2x80x3, R9xe2x80x3, R10xe2x80x3, X, Y, k, m and n have the same meanings as above.
The styryl compounds of formula (I) can be produced by various known methods. For producing these, for example, mentioned are the following three methods.
Method 1
A phosphonate of a general formula (a): 
wherein k, m and n each are 0 or 1, and (k+m+n)xe2x89xa71;
R1 to R12, R3xe2x80x2, R4xe2x80x2, R9xe2x80x2, R10xe2x80x2, R31xe2x80x3, R4xe2x80x3, R9xe2x80x3 and R10xe2x80x3 have the same meanings as above; and R represents an alkyl group having from 1 to 4 carbon atoms, or a phenyl group,
is condensed with a carbonyl group of a general formula (b): 
wherein X and Y have the same meanings as above, in the presence of a base through Wittig reaction or Wittig-Horner reaction to give styryl compounds of formula (I).
Method 2
A dialdehyde compound of a general formula (c): 
wherein k, m and n each are 0 or 1, and (k+m+n)xe2x89xa71;
and R1 to R12, R3xe2x80x2, R4xe2x80x2, R9xe2x80x2, R10xe2x80x2, R31xe2x80x3, R4xe2x80x3, R9xe2x80x3 and R10xe2x80x3 have the same meanings as above,
is condensed with a phosphonate of a general formula (d): 
wherein R, X and Y have the same meanings as above, in the presence of a base through Wittig reaction or Wittig-Horner reaction to give styryl compounds of formula (I).
As the reaction solvent for that condensation, preferred are hydrocarbons, alcohols, and ethers. Specific examples of the solvent are methanol, ethanol, isopropanol, butanol, 2-methoxyethanol, 1,2-dimethoxyethane, bis(2-methoxyethyl)ether, dioxane, tetrahydrofuran(THF), toluene, and xylene. Also preferably employed are dimethylsulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, 1,3-dimethyl-2-imidazolidinone, etc. Especially preferred are tetrahydrofuran and dimethylsulfoxide.
As the condensing agent, preferred are sodium hydroxide, potassium hydroxide, sodium amide, sodium hydride, n-butyl lithium, and even alcoholates such as sodium methylate, potassium t-butoxide, etc. Especially preferred are n-butyl lithium and potassium t-butoxide.
The reaction temperature varies, depending on the starting compounds to be reacted, and therefore cannot be defined indiscriminately. In general, however, it may be widely from 0xc2x0 C. to about 100xc2x0 C. Especially preferably, the reaction temperature falls between 0xc2x0 C. and room temperature.
Method 3
A bromide of a general formula (e): 
wherein X, Y, R1, R2, R7, R8, and also R5, R6, R11 and R12 have the same meanings as above,
is reacted with Mg to give a Grignard reagent, which is coupled with a dibromoarylene compound of a general formula (f): 
wherein k, m and n each are 0 or 1, and (k+m+n)xe2x89xa71;
and R3, R4, R9, R10, R3xe2x80x2, R4xe2x80x2, R9xe2x80x2, R10xe2x80x2, R3xe2x80x3, R4xe2x80x3, R9xe2x80x3 and R10xe2x80x3 have the same meanings as above,
in the presence of a metal catalyst to give styryl compounds of formula (I).
As the metal catalyst for the coupling, employable is a transition metal complex catalyst. Preferred are nickel catalysts and palladium catalysts, which include, for example, NiCl2 (dppp) (Tokyo Kasei), [NiCl2(PPh3)2], and also PdCl2 (dppf), and Pd(PPh3)4.
As the reaction solvent, employable is any of dewatered diethyl ether, THF, di-n-propyl ether, di-n-butyl ether, di-i-propyl ether, diethylene glycol dimethyl ether (diglyme), dioxane, dimethoxyethane (DME), etc.
Preferred are diethyl ether and THF.
Specific examples (1) to (61) of the styryl compounds for use in the invention are mentioned below, which, however, are not intended to restrict the scope of the invention. 
In the invention, one or more of those organic host compounds may be used either singly or as combined.
The fluorescent substance which is the other component constituting the organic blue-emitting layer is doped into the organic light-emitting layer in order to improve the efficiency of the organic EL device and to prolong the life thereof. The fluorescent substance is not specifically defined, provided that it can emit light in response to the recombination of holes and electrons, and may be any known fluorescent dye. However, it is important that the fluorescent substance is so selected that its energy gap is smaller than the energy gap of the organic host compound. The fluorescent substance includes, for example, stilbene derivatives, tristyrylarylene derivatives, and distyrylarylene derivatives (see Japanese Patent Application Laid-Open No. 5-129438). In the invention, one or more such fluorescent substances can be used either singly or as combined.
The organic host compound and the fluorescent substance are suitably selected and combined to give the organic blue-emitting layer, in which the efficient energy transfer from the organic host compound to the fluorescent substance is realized to attain the improvement in the efficiency of the organic EL device and the prolongation of the life thereof. In that preferred case, it has been found that the profile of the EL spectrum quite agrees with that of the fluorescence spectrum of the fluorescent substance within a range of xc2x110 nm with respect to the fluorescence peak wavelengths and the individual peak wavelengths of the vibronic structure. The fluorescence spectrum of the fluorescent substance is measured in a solution of the fluorescent substance as dissolved in a non-polar solvent such as toluene. When the profile of the EL spectrum quite agrees with that of the fluorescence spectrum of the fluorescent substance, it is herein referred that the EL device has xe2x80x9cmonomeric light-emitting abilityxe2x80x9d.
In the organic EL device of the invention having the monomeric light-emitting ability, it is meant that the initial state of EL corresponds to the excited monomeric state of the fluorescent substance in the organic light-emitting layer.
In order to improve the luminous efficiency of the organic EL device, it is effective to increase the fluorescence quantum efficiency of the organic host compound in the device. In the invention, therefore, that quantum efficiency must be not smaller than 0.3. The fluorescence quantum efficiency of the organic host compound is measured in a thin film of the substance, and is different from that as measured in its solution.
The organic EL device of the invention must retain a monomeric blue-emitting ability. Therefore, the organic host compound and the fluorescent substance are selected such that the interaction between the organic host compound and the fluorescent substance is absent and the interaction between the organic host compound and the adjacent compound layers is absent. As for blue-emitting organic host compounds, the interaction between the organic host compound and the fluorescent substance existing in the emitting layer and the interaction between the organic host compound and the adjacent organic compound layers are often large in many cases, in those cases the state having energy smaller than the energy of an excited state of the fluorescent substance is formed (an exciplex) . As a result, in those cases, the energy of the excited state of the fluorescent substance is transferred to the state having smaller energy, whereby the EL device gives a broad emission spectrum which, being different from the fluorescence spectrum of the monomeric emission from the fluorescent substance, has peaks at longer wavelengths. Even if the EL devices of those cases could produce monomeric emission in the initial stage of their driving, the light as emitted by them will often become different from monomeric one while the devices are continuously driven for long. Such EL devices that could not retain the ability of monomeric emission shall have an extremely short life.
Therefore, in the organic EL device of the invention, it is extremely important to select the combination of the organic host compound and the fluorescent substance so that the device can retain the ability of monomeric emission.
In addition, all the organic compound layers constituting the organic EL device of the invention must have a glass transition temperature of not lower than 75xc2x0 C., in order to make the device have good heat resistance. Moreover, in order to further improve the storage stability of the device at 75xc2x0 C., the stability of the interfaces between the organic light-emitting layer and the adjacent organic compound layers is an important factor. Therefore, the organic compound layers adjacent to the organic light-emitting layer must have a glass transition temperature of not lower than 105xc2x0 C. If satisfying those requirements, the organic EL device is free from the change in the color to be emitted by it and from the reduction in the efficiency of the device, while maintaining its good properties for long, even though stored in a high-temperature atmosphere at 75xc2x0 C.
The layer constitution of the organic EL device of the invention is not specifically defined, and may be any desired one. Basically, however, the organic blue-emitting layer is sandwiched between a pair of electrodes (positive electrode and negative electrode), in which are optionally provided a hole injection and transportation layer and an electron injection layer. Those are formed on a transparent substrate, through which the light emitted is seen. Examples of the organic EL device having that layer constitution are mentioned below.
(1) Positive electrode/organic light-emitting layer/negative electrode
(2) Positive electrode/hole injection layer/organic light-emitting layer/negative electrode
(3) Positive electrode/organic light-emitting layer/electron injection layer/negative electrode
(4) Positive electrode/hole injection layer/organic light-emitting layer/electron injection layer/negative electrode
(5) Positive electrode/hole injection layer/hole transportation layer/organic light-emitting layer/electron injection layer/negative electrode
In these structures, the organic compound layers such as the hole injection layer, the hole transportation layers and the electron injection layer are not specifically defined, provided that they satisfy the requirements for their glass transition temperature.
The organic EL device of the invention is optionally provided with a hole injection and transportation layer, which functions to inject holes thereinto from the positive electrode and to transport them into the light-emitting layer. Preferably, the hole injection and transportation layer has a hole mobility of not smaller than 10xe2x88x926 cm2/Vxc2x7s in an electric field of from 104 to 106 V/cm. If desired, the organic EL device may be provided with a laminate of a hole injection layer and a hole transportation layer.
The material to be in the hole injection and transportation layer may be selected from, for example, compounds of a general formula (III): 
In formula (III), Q1 and Q2 each represent a group having a nitrogen atom and at least three carbon rings (of which at least one is an aromatic ring such as a phenyl group), and these maybe the same or different; G represents a cycloalkylene group, an arylene group, or a linking group comprising a carbon-carbon bond.
In the organic EL device of the invention, the hole injection layer and the hole transportation layer that are directly adjacent to the organic light-emitting layer must have a glass transition temperature of not lower than 105xc2x0 C. Therefore, the materials of those layers are preferably selected from oligomer amines of the compounds of formula (III) comprising three or more arylamines as bonded in a linear or branched manner.
The compounds of that type are, for example, represented by the following general formula (IV): 
wherein R13 to R17 each represent an alkyl group, an alkoxy group or a phenyl group, and may be the same or different, the phenyl substituent being optionally condensed with the group on which it is substituted to give a naphthyl group.
Specific examples of the compounds are mentioned below. 
The layers may comprise one or more of those compounds either singly or as combined.
The organic EL device of the present invention is optionally provided with an electron injection layer (electron injection and transportation layer), which functions to transfer the electrons as injected thereinto from the negative electrode to the organic light-emitting layer, and may comprise any known conventional electron-transmitting compound. For example, the material to be in the layer is preferably selected from metal complexes of 8-hydroxyquinoline or its derivatives, or oxadiazole derivatives.
As examples of metal complexes of 8-hydroxyquinoline or its derivatives, mentioned are metal chelate oxanoid compounds containing chelates of oxine (generally, 8-quinolinol or 8-hydroxyquinoline), etc. Compounds of that type all have a glass transition temperature of not lower than 105xc2x0 C.
The electron injection layer may comprise one or more of those compounds either singly or as combined.
Preferably, the organic EL device of the invention having the constitution mentioned above is supported by a substrate, and the substrate in this use is not specifically defined. Any ordinary substrate for conventional organic EL devices is employable herein. For example, employed is any of glass or transparent plastics.
The positive electrode constituting the organic EL device of the invention is to inject holes into the device. For this, preferred are electrode materials having a large work function (not smaller than 4 eV), such as metals, alloys, electroconductive compounds and their mixtures. Specific examples of such preferred electrode materials are metals such as Au, and electroconductive transparent materials such as CuI, ITO (indium tin oxide), SnO2, ZnO, etc. The positive electrode can be formed, for example, through vacuum vapor deposition or sputtering of such an electrode material to give a thin film. For light emission through the electrode, it is preferred that the electrode has a transmittance, relative to the light emitted, of not smaller than 10%, and that the sheet resistance of the electrode is not larger than hundreds of ohms per square (xcexa9/xe2x96xa1).
The thickness of the electrode film may be generally between 10 nm and 1 xcexcm, preferably between 50 and 200 nm, depending on the material of the electrode.
The negative electrode constituting the organic EL device of the invention is to inject electrons into the device. For this, preferred are electrode materials having a small work function (not larger than 4 eV), such as metals, alloys, electroconductive compounds and their mixtures. Specific examples of such preferred electrode materials are sodium, sodium/potassium alloys, magnesium, lithium, magnesium/copper mixtures, magnesium/silver alloys, aluminium/lithium alloys, Al/Al2O3 mixtures, indium, rare earth metals, etc. The negative electrode can be formed, for example, through vacuum vapor deposition or sputtering of such an electrode material to give a thin film. For light emission through the electrode, it is preferred that the electrode has a transmittance, relative to the light emitted, of not smaller than 10%, and that the sheet resistance of the electrode is not larger than hundreds of ohms per square (xcexa9/xe2x96xa1). The thickness of the electrode film may be generally between 10 nm and 1 xcexcm, preferably between 50 and 200 nm, depending on the material of the electrode.
Now, preferred embodiments of producing the organic EL device of the invention are referred to hereinunder. First, a thin film of a desired electrode material, for example, a positive electrode material is formed on a suitable substrate through vapor deposition or sputtering to have a thickness of from 50 to 200 nm. This is formed a positive electrode on the substrate. Next, thin films of a hole injection layer, a hole transportation layer, an organic blue-emitting layer, and an electron injection layer are formed on the positive electrode.
To form those thin films, for example, employable is any of spin-coating, casting or vapor deposition. Preferred is vacuum vapor deposition, through which uniform films with few pin holes are easy to obtain. For the vapor deposition to form those thin films, the condition varies, depending on the type of the compound to be vaporized for the deposition, and the intended crystal structure and association structure of the molecular film to be deposited, but is preferably such that the boat heating temperature falls between 50 and 400xc2x0 C., the vacuum degree falls between 10xe2x88x926 and 10xe2x88x923 Pa, the deposition rate falls between 0.01 and 50 nm/sec, the substrate temperature falls between xe2x88x9250 and 300xc2x0 C., and the film thickness falls between 5 nm and 5 xcexcm.
After the formation of those layers, a thin film of a negative electrode material is formed thereover, for example, through vapor deposition or sputtering to be a negative electrode having a film thickness of from 10 nm to 1 xcexcm, preferably from 50 to 200 nm. Thus is produced the intended organic EL device. To produce the device, the order of forming the electrodes and the layers may be reversed.
Where a direct current voltage is applied to the organic EL device thus produced in that manner, a voltage of from 3 to 40 V or so may be applied thereto with its positive electrode being charged to be plus (+) and its negative electrode to be minus (xe2x88x92), whereby the device emits blue. Even if the same voltage is applied to the device in the reversed manner relative to the polarity of the electrodes, the device emits no light. Where an alternating current is applied to the device, the device emits light only when its positive electrode is charged to be plus (+) and its negative electrode to be minus (xe2x88x92). The wave mode of the alternating current to be applied to the device may be any desired one.
Now, the invention is described in more detail with reference to the following Examples, which, however, are not intended to restrict the scope of the invention.